I was looking at some letters today and thought this might be useful to you-
Mod 1926 / 6-1/2" / Blue Ser # 54849 shipped 4-1-40 to Frank T. Budge Co - Miami
Mod 1926 / 6-1/2" / Blue Ser # 54832 (17 #'s lower)shipped 1-18-41 (9-1/2 mos later) to Frank T. Budge Co - Miami
The 6-1/2's were not hot sellers, so that could account for the wide dates IF a batch had been made in this number range.
Interesting that a Miami dealer seemed to move them.
Here is another one with a slightly lower serial number that shipped between your two and went to the other end of the east coast. But this one is a Target!
Mod. 1926, sn 54726, blue, 6.5" bbl. shipped 9-16-1940 to John J. Tobler Co., Union City, NJ.
Well I have a 5" 1926 serial 55900 that letters 6/39. This is about 900 units AFTER Lee's two guns and it shipped 10 months earlier than the first one went out. Unless there was a special police order to be filled I suspect the long HDs were very slow movers.
I believe this is the usual explanation for delays in shipping. The gun just wasn't popular (any more). My 4" HD has a serial suggesting a 1956 date but letters as 1960. Set in inventory for 4 years. I think the Combat Magnum killed the HD sales.
I think that you are correct that the 6.5" 3rd Models were slow movers. That also extended to those made post war. I also have a 6.5" M1926 (S 6334x) that shipped on July 18, 1946 to Sports Inc., Chicago and in Roy's letter he states: "Six guns in shipment, one with 6.5" bbl., five with 5" bbl."
I would imagine that ratio was no lower throughout the production.